Palladium-catalysed CC coupling reactions (for example Suzuki coupling, allyl substitution, the Heck reaction) play an important role in preparative organic chemistry. In particular, the toleration of many functional groups, which makes an expensive protective group technique superfluous, make CC coupling reactions particularly attractive for fine chemistry and pharmaceutical chemistry.
Homogeneous catalysts for CC coupling reactions are known. On a production scale, with these catalysts it is essential to remove the palladium from the reaction solution in a further process step. Since in most cases the noble metal palladium is used as catalytically active metal, it must be separated off as quantitatively as possible. On the one hand, the expensive noble metal can be recovered in this way, and on the other hand contamination of the product with noble metal is avoided.
Numerous disadvantages are associated with the use of homogeneous catalysts for CC coupling reactions according to the prior art. After the reaction has taken place, the noble metal must be separated off from the reaction mixture in an expensive manner, which complicates the preparation process. Homogeneous catalysts are in general composed of the noble metal and ligands which are in a particular stoichiometric ratio with respect to one another. These ligands also make purification of the product expensive. Inert gas conditions (nitrogen or argon atmosphere) are very often required, so that the catalyst is not destroyed before or during the reaction.
It is known that CC coupling reactions can be carried out with heterogeneous catalysts. Augustine (R. L. Augustine, S. T. O'Leary, J. Mol. Catal. 1995, 95, 277–285) describes, for example, the reaction of aromatic carboxylic acid chlorides with butyl vinyl ether to give 2-butoxyethenylbenzenes. Eisenstadt (EP 509 426) describes the preparation of UV absorbers with the aid of heterogeneous Pd/C catalysts.
Numerous problems are also linked with the use of heterogeneous catalysts for CC coupling reactions according to the prior art. When halogenoaromatics are employed in, for example, the Heck reaction, an undesirable dehalogenation is often observed.
In direct comparison between homogeneous and heterogeneous catalyst systems, the heterogeneous systems show lower activities than the homogeneous systems.
A very important aspect, which is often not considered in the literature, is the detachment of the noble metal from the support during and/or after the reaction. Where detachment of the noble metal from the support has been investigated at all, this is often so high that contamination of the product takes place. According to the current prior art, it is very expensive or as a rule even impossible to separate off the noble metal completely.